Spectral evolution of pulse structures in gamma-ray bursts

Abstract

The Hard X-Ray Burst Spectrometer (HXRBS) and the Gamma-Ray Spectrometer (GRS) on the Solar Maximum Mission satellite have independently monitored cosmic gamma-ray bursts since launch in 1980 Feb-ruary. Ten relatively intense events detected by the GRS (four detected by HXRBS) have been studied in an attempt to reveal a basic pattern of spectral variability in gamma-ray bursts. In each of these bursts we find pulses > 1 s in duration which exhibit a hard-to-soft spectral evolution. We find no evidence for soft-to-hard evolution in any of the pulses. Using the HXRBS data for energies above 144 keV and GRS data above 1 MeV, we demonstrate that this hard-to-soft evolution is not due to time-varying absorption features at energies below 100 keV. Details of the spectral evolution of pulses are presented for two events, 1982 March 1 and November 4. We also show that the KONUS observations of Golenetskii and colleagues, updated in 1983, for which a detailed correlation of intensity and spectral hardness was reported, are actually consistent with our findings if different assumptions are made. Several possible explanations for spectral softening of burst pulses are suggested.